Being a former Marine machine-gunner, I don't have a lot of experience with the M60, but I have a lot of experience with the M240G, which is similar in terms of operation. The M240G, just like the M60, fires from an open bolt. The bolt is locked to the rear when the trigger is not being pressed; the trigger pull releases the bolt, it slams forward, chambers a round, fires it, then the bolt goes back and if the trigger isn't held anymore the bolt locks back to the rear. So if the chamber is so hot that it would cause a round to cook off, it doesn't really matter because there is no round in the chamber in between bursts.

So on an open-bolt belt-fed machine-gun it is impossible to have a runaway gun because of a cook-off; in fact, it's impossible to even have a cook-off in the normal sense of the word because every time a round is slammed into the chamber it's immediately fired anyway. In between bursts, the ammo is not in the chamber, it's on the feed tray. So if the gun was so hot that it caused the round to explode it wouldn't go down the barrel, it would just explode on the feed tray and the bolt would probably stay locked to the rear; and even if the bolt went forward there would be no live rounds left on the feed tray to chamber.

On an open-bolt belt-fed machine-gun, a runaway gun is caused by the sear breaking (or just wearing down) and not catching the bolt back after you let go of the trigger. It usually happens when the gun is hot because that means you just put a lot of stress on the sear.